"Carbon nanotubes are superior thermal conductors by themselves. But, that doesn't mean they will exhibit the same level of high conductivity when integrated into other materials," says Pawel Keblinski, assistant professor of materials science and engineering and head of Rensselaer's research team. His team's research is published in this month's issue of Nature Materials.
A global team of researchers was optimistic when a one-percent fraction of carbon nanotubes was added to epoxy and other organic materials, and the thermal conductivity of the newly created composites increased two- or threefold. But, using conventional engineering estimates, Keblinski noted that the composites' conductivity should have had 50-fold increases.
Why such disparity between the experiment and the expectations?
"Atoms forming stiff carbon nanotubes vibrate at much higher frequencies than the atoms in the surrounding material. This leads to high interfacial resistance for the heat flow between the tubes and the other elements," Keblinski says.
Energy exchange between two different elements is immediate and plentiful when frequencies in both are similar. Interfacial resistance happens when the frequencies are different, and the heat energy has a difficult time taking the leap from one element to the next.
To test the magnitude of the problem, Keblinski and his Rensselaer collaborators performed computer simulations on a model nanotube composite. Meanwhile, another research group headed by David Cahill at the University at Illinois at Urbana Champaign, heated real carbon nanotubes with a laser.
From the rate of cooling, in both the simulation and the physical experiment, the researchers derived the value of the interfacial resistance. In both instances, they found the resistance is so high that it limits the thermal conductivity of the nanotubes
One way to reduce the interfacial resistance in such nanocomposites is to induce a stronger bond between the nanotube and other materials to make it easier for heat to cross from one element to the next. However, extensive bonding may distort the original nanotube structure that allows the tubes to be a superconductor of heat in the first place.
Still, Keblinski is optimistic about the use of carbon nanotubes to improve insulating materials. "By adding a small fraction of carbon nanotubes to such materials, we can still increase the thermal as well as electrical conductivity. So, although we may have to lower our expectations, we have not given up hope quite yet that nanotubes will improve materials for a number of applications," Keblinski says.
Rensselaer Polytechnic Institute, founded in 1824, is the nation's oldest technological university. The school offers degrees in engineering, the sciences, information technology, architecture, management, and the humanities and social sciences. Institute programs serve undergraduates, graduate students, and working professionals around the world. Rensselaer faculty are known for pre-eminence in research conducted in a wide range of research centers that are characterized by strong industry partnerships. The Institute is especially well known for its success in the transfer of technology from the laboratory to the marketplace so that new discoveries and inventions benefit human life, protect the environment, and strengthen economic development.